Methane reforming is a chemical process that results in the production of pure hydrogen from natural gas. There are two basic types of natural gas reforming technologies: steam methane reforming (SMR) and autothermal reforming (ATR). Both methods work by exposing natural gas and steam to a catalyst (typically nickel) at high temperature.
Conventional steam methane reforming (SMR) uses an external source of hot gas to heated tubes in which a catalytic reaction takes place that converts steam and lighter hydrocarbons—such as natural gas or refinery feedstock—into hydrogen and carbon oxides (i.e., syngas).
During a conventional SMR process, the following two reactions take place concurrently:CH4+2H2O→CO2+4H2 CH4+H2O→CO+3H2 A water-gas shift reaction is then performed using steam to convert carbon monoxide to carbon dioxide and generate additional hydrogen. This is typically followed by a pressure-swing adsorption (PSA) step to purify the hydrogen.
Because the above two reactions are both endothermic, residual, unreacted tail gas from the PSA process is not wasted, but finds a ready use to heat the reformer to drive the reaction.
In autothermal reforming (ATR), oxygen is added to the process, resulting in the additional reactions:CH4+O2→CO2+2H2 CH4+½O2→CO+2H2 The above reactions are exothermic and—provided that enough oxygen is added to balance the two endothermic reactions out—can result in an overall adiabatic process. Therefore, no additional heat source is needed. Although requiring oxygen feed, the ATR has the advantage that it is more compact and less expensive to build than the SMR, because the heat transfer surface required is considerably less with an adiabatic process.
A basic process schematic for a conventional ATR process is shown in FIG. 1. According to this process, natural gas, 101, steam, 102, and oxygen, 103, are introduced into an autothermal reformer, 104. The resulting gas, 105, comprising carbon dioxide, carbon monoxide, hydrogen, unreacted natural gas and excess steam, is cooled and sent to a shift reactor, 107. The shift reaction is performed through the addition of steam, 106, to convert the carbon monoxide to carbon dioxide. The resulting gas mixture, 108, is cooled in step, 109, to condense out water, 110. The resultant gas mixture, 111, is passed to a hydrogen separation unit, 112 (typically, a pressure swing adsorption unit) to separate hydrogen, 113. Tail gas, 114, contains mostly carbon dioxide, together with some carbon monoxide, hydrogen, inerts, and unreacted methane.
Tail gas from the ATR process described above is not pure carbon dioxide, so if it is desirable to recover and/or capture carbon dioxide for sequestration or some other purpose, the tail gas stream needs to be purified. Not only does the carbon dioxide have to be purified, but the impurities should be recovered in fairly concentrated form to minimize carbon dioxide loss, i.e., maximize carbon dioxide recovery.
The process of the subject invention achieves this objective in a different, more efficient way. By using the process of the invention, substantially all of the carbon dioxide is recovered; losses of methane and carbon monoxide are minimized; and steam consumption in the reformer is drastically reduced.